This invention relates to optical coatings and surfaces and methods of making optical surfaces, and in particular, to inorganic, optical, black, silicate coatings and surfaces and methods of making such surfaces.
Optical coatings and surfaces are known and are used in instances where low light reflectivity, or conversely high light absorptivity, is required, that is, surfaces which absorb a substantial portion of the electromagnetic radiation, especially in the solar spectrum, to which they are exposed. The highest absorptivity is achieved with black coatings and surfaces. Uses for optical coatings and surfaces include the interiors of solar telescopes, binoculars, camera bodies, and the like, where reflected radiation will interfere with the radiation being observed or measured and solar panels where radiation, such as solar energy, is absorbed for conversion into some other form of energy, such as heat or electricity.
There has been much development work in the area of providing and developing metal bodies having a surface finish such that they absorb an extremely high percentage of ultra-violet, visible and infrared radiation, and therefore only a very low percentage of such radiation is reflected therefrom. Some of the development work has involved processes for treating the surface of the body involved to improve its optical character, and some has involved coatings and coating processes which result in improved optical characteristics for the surface of the body.
Among the known coatings for producing optical, especially black, surfaces are the organic black coatings and the so-called high temperature black coatings. The most well known of the organic coatings is the 3M (Minnesota Mining and Manufacturing Company) Nextel black velvet coating or paint which has a composition by weight of approximately 16% pigment and 84% organic vehicle (basically a polyester base material). The pigment comprises approximately 20% carbon black and approximately 80% silicon dioxide. This material is commonly used for coatings in optical instruments such as telescope tubes, camera housings, vacuum chamber walls, etc.
In addition to the 3M Nextel black velvet paint, another well-known high absorbent of visible and infrared radiation is Parson's black. Parson's black consists of an alkyd lacquer containing carbon black. The carbon black, which is a powdery material, is adhered to the surface of a body to give that surface a high radiation absorption capability. Parson's black is, in general, a better visible and infrared absorbent than 3M Nextel black velvet paint.
Other optically black organic coatings have been developed, but they differ basically in the type of vehicle employed, such as epoxy and acrylic base coatings. None of the other organic black coatings, to applicant's knowledge, achieve the high degree of absorbency of visible and infrared radiation as does the 3M Nextel black velvet paint and Parson's black.
The 3M Nextel black velvet paint and Parson's black, both of which, as noted above, are known for their high absorption capability of visible and infrared radiation, have a substantial shortcoming in their lack of durability. The 3M black velvet paint is subject to chipping after moderate temperature exposure and hydrocarbon outgassing, both of which detrimentally affect the desirability of the product. In addition, because the organic binder will degrade at elevated temperatures, the organic coatings are restricted to low temperature applications. Further, Parson's black, which contains a relatively high percentage of powdery carbon black, also lacks durability, being very easily removed from any surface on which it is applied.
The so-called high temperature black coatings are not entirely free from the problems of organic binders since they basically comprise an organic material having inorganic components which are deposited as a residue. An example is the silicone resin based "high temperature" coating, which is commercially available. The inorganic surface is formed by coating a silicone resin on the substrate which is to have the optical surface, heating the coating to about 600.degree. F. to 1000.degree. F. to burn-off the organic binder which leaves an inorganic residue, and then heating the residue to in excess of 1000.degree. F. to sinter the residue and thus form an inorganic layer on the substrate. Such a coating is inorganic and so will avoid generally the off-gassing problems associated with organic coatings, but such a coating process requires a large amount of expensive high temperature processing equipment, as well as processing steps, and if not properly heat treated, an organic residue may remain. Further, in order to avoid the formation of heat scale, which is associated with ferrous alloys during the high temperature treatment step, e.g., on the inside of tubes which are being optically coated, a means is required to protect the inside of the tubes, such as an inert gas purge inside the tubes or the like treatment, which only adds to the complexity and expense of the process.
Silicate coatings, such as sodium and potassium silicate, are well known for such purposes as high temperature resistance and corrosion resistance. Silicate coatings normally are not noted for their optical qualities, and in fact, are considered to have only average absorptivity or reflectivity levels. Often, silicate coatings are used as a primer, i.e., a protective coating which precedes the ultimate surface coating of paint. Further, while silicate coatings are inorganic and thus do not suffer from the problems of organic coatings, they are known, depending upon the formulation, to suffer from problems of durability and moisture resistance. Examples of silicate coatings are U.S. Pat. Nos. 2,076,183; 2,711,974; 3,416,939; 3,615,282; 3,620,791; and 3,769,050; and British patent specification No. 643,345.
U.S. Pat. No. 2,076,183 is of particular note because it discloses a heat resistant, permanent black, sodium silicate finish. But, such a coating would not be considered an optical black coating in that it would not have a sufficiently high solar absorptivity, especially as compared to, e.g., 3M Nextel velvet black. Thus the black of U.S. Pat. No. 2,076,183 would only be a general purpose black.
Thus, a need exists for an optical coating and surface which has a high absorptivity, and thus, a low reflectance of electromagnetic radiation, especially in the solar spectrum, and does not suffer from problems such as off-gassing or chipping or high temperature degradation.